1. Field of the Invention
This invention relates to a catalyst for purifying exhaust gases which can reduce with high efficiency nitrogen oxides, carbon monoxide and hydrocarbons, the noxious components of exhaust gases from internal combustion engines, etc.
2. Description of the Prior Art
Various catalysts have been suggested to date for reducing the noxious components of such exhaust gases as mentioned above. Catalysts containing platinum, palladium or rhodium as a catalyst ingredient supported on an alumina carrier are considered to have a relatively superior purifying activity.
However, catalysts containing a single metal of platinum or palladium as a catalyst ingredient, i.e., a platinum (Pt) catalyst and a palladium (Pd) catalyst, have a poor purifying activity, especially in reducing nitrogen oxides (NO.sub.x). Therefore, catalysts containing a mixed metal of platinum or palladium with rhodium as a catalyst ingredient, i.e., a platinum-rhodium (Pt-Rh) catalyst and a palladium-rhodium (Pd-Rh) catalyst have been put to practical use. Nevertheless, the amount of rhodium present on the earth is said to be from one tenth to one fifth of the amount of platinum. Therefore, the use of rhodium should be reduced to as small an amount as possible.
Thus, the various conventional catalysts which have been suggested to date are still unable to meet the requirements for the purification of motor vehicle exhaust gases because they cannot reduce the aforesaid three noxious components with a high efficiency, and they cannot be obtained at an economically low cost.
A need therefor continues to exist for a catalyst for the reduction of nitrogen oxides, carbon monoxide and hydrocarbons in the exhaust gases from internal combustion engines, which exhibits high efficiency for said reduction and which can be obtained at an economically low cost.